Field of the Invention
The present invention relates generally to a system for protecting electrical machines from damage caused by residual voltage effects following a momentary interruption in the electrical power supply, and more particularly to a system for allowing quick reconnection to a power supply after dissipating the residual voltage in the machine by short circuiting the stator windings.
When the electrical power supply to an electrical machine is interrupted, a residual voltage which decays exponentially with time will be produced within the machine. Induction motors and generators produce such a voltage at their terminals, and synchronous motors and generators operate as induction machines following interruption of current in the DC field windings. Synchronous machines, of course, have somewhat longer open circuit and short circuit time constants.
The initial residual voltage magnitude in an induction machine (or in a synchronous machine with its DC field interrupted) may be nearly equal to the magnitude of the source voltage. The residual voltage decays exponentially to approximately thirty-seven percent of its initial value in a period of time which is equal to the machine's open circuit time constant. The open circuit time constant may vary from approximately one-half second for smaller machines to three or four seconds for fairly large machines, or even longer in the case of large synchronous machines.
If the power supply to an electrical machine is interrupted and quickly re-established, the machine may be subjected to a voltage equal to its residual voltage added vectorially to the voltage of the re-established oncoming power supply. If the resulting voltage across the machine is greater than approximately 125 to 130 percent of the rated voltage of the machine, the resulting high current and torque may result in significant damage occurring to the machine.
It will be recognized that it is highly desirable to resume operation as soon as possible; indeed, fast transfer of medium voltage auxiliaries is a standard feature of most major U.S. power plants. However, due to residual voltage, particularly in larger electrical machines, rapid reclosing of the utility switch to restore system power and operation is seldom a possibility. Following a tripping occurrence such as a self-clearing short circuit in a plant with large electrical machines, it is not possible to quickly re-establish power to the plant even though the condition which caused the interruption is no longer present.
Similarly, if large electrical machines are in use on the line it is not possible to rapidly switch from an interrupted or failed primary source of power to a secondary source of power due to the presence of residual voltages which effectively prevent such a rapid transfer. It is apparent that in either the case of a momentary interruption in the power source due to a self-clearing fault or in the case of a transfer to an auxiliary power source, large machines on the line will effectively cause a line shutdown, which is a very unwelcome consequence potentially occurring relatively frequently.
In the past three switching techniques have been utilized to avoid machine damage: conventional fast transfer, in-phase transfer, and delayed transfer. The first technique, conventional fast transfer, is possible only under the condition that it is possible to switch from one source to another so fast that residual voltage of the machine will not drift too far out of phase with the auxiliary voltage source. Fast transfer systems are used extensively in power plants for transfer of the medium voltage auxiliary bus from the normal source to a standby or emergency source without affecting plant operations.
In order for fast transfer to be safely used, it is necessary for several conditions to be met. First, the machines and accompanying mechanical equipment must always be in operation and have high and fully predictable inertia, generally an inertia constant H which is greater than two (2.0). Secondly, the two sources must always be in synchronism, having almost exactly the same phase angle. An additional limitation is imposed on the switchgear, which must be of the high speed stored energy type to function properly in making the fast transfer. It is also important to realize that fast transfer is only initiated in the event of a supply transformer failure.
Other types of source interruptions, which occur more frequently than supply transformer failures, are likely to introduce uncertainties as to the transfer time or the rate of machine deceleration. These conditions and restrictions relegate conventional fast transfer switching to a relatively insignificant role. Attempts to utilize a fast transfer system in applications beyond the narrow range defined above incur significant risk to both the electrical machines and the driven equipment.
The second switching technique utilized in the past is in-phase transfer, which is substantially similar to conventional fast transfer. In-phase transfer differs from fast transfer in that it utilizes a bus transfer relay to switch to an alternate voltage source at precisely the right moment, thereby assuring that the phase angles of the residual machine voltage and the oncoming line voltage are identical or within acceptable limits. The simplest form of such a bus transfer relay is a phase angle relay, but in such a system safety is dependent on constant bus loading, constant machine loading, and the presence of both large and predictable load inertias, conditions which are usually not attainable in practice.
A more sophisticated but complex transfer relay system will adjust its performance to match existing loading conditions. While this is more practical in that it allows for variable loading of the bus and machines, it has one significant disadvantage in that it requires additional time for detection of loading conditions. The additional delay restricts such a system to high inertia systems with a slip at transfer generally not exceeding 8.3 percent. Only machines having an effective inertia constant H in the range of one (1.0) to two (2.0) or greater will meet this requirement and allow the use of such a system.
The third technique of switching, and the most popular technique to date, is delayed transfer, which utilizes a timer or a residual voltage relay to delay switching to an alternate voltage source until motor residual voltage has safely decayed to 25 or 30 percent of machine rated voltage. While delayed transfer is essentially risk free, transfer is so slow that it inevitably interrupts process operations. In addition, motors generally can not be re-accelerated simultaneously following a slow transfer in which motor speeds have fallen so low that inrush currents approach motor locked rotor values.
In such a case stalling would occur due to depressed voltage. Because of this inevitable consequence, delayed transfer systems typically provide for re-accelerating only those drives necessary for an orderly shutdown. Drives are sometimes then restarted in orderly groups, and full plant operations are usually restored in from one to five minutes. Such shutdowns are generally costly, and all too frequently result in costly process upsets.
It is therefore apparent that the three known switching techniques are not at all satisfactory in most cases. It is desirable to have an alternative switching technique which overcomes the disadvantages inherent in conventional fast transfer, in-phase transfer, and delayed transfer. Such a system would allow for fast switching without the disadvantages presented by the techniques described above.